1. Field of the Invention
The present invention is directed to a method for generating a two-dimensional image from a 3D dataset of a tomography apparatus for the medical examination of a patient, wherein the 3D dataset contains picture elements arranged is three-dimensional grid, and wherein an evaluation surface proceeding through the grid is defined and the picture elements along the evaluation surface are employed for constructing the two-dimensional image. The invention is also directed to a medical tomography apparatus with which a 3D dataset is generated in as picture elements in a three-dimensional grid during the examination of a patient, having an input unit for defining an evaluation surface proceeding through the grid, and having an evaluation unit for constructing a two-dimensional image from the 3D dataset using the picture elements along the evaluation surface. The invention is also directed to a data carrier for such a method.
2. Description of the Prior Art
Spatial tomogram sequences with isotropic resolution, i.e. with the same spatial resolution in the x, y and z direction, or with approximately isotropic resolution can be generated with medical tomographic image acquisition methods. Such image acquisition methods are involved, for example, in computed tomography (spiral CT), in three-dimensional magnetic resonance tomography, and in 3D ultrasound examinations. The tomogram sequences respectively arising in the acquisition are referred to as volume data. They can be interpreted as a three-dimensional, regular grid, wherein tissue properties at every intersecting point are represented by a gray scale values. Alternatively, the volume data can be imaged as a large cuboid composed exclusively of identical cuboids. An elementary cuboid is called a voxel, has a gray scale value, and covers a certain tissue volume. Volume data, i.e. 3D datasets, also can be generated by conventional x-ray devices, for example by a C-arm device.
Two-dimensional images must be acquired for visualizing the volume data, i.e. the 3D dataset, for the viewer, for example a physician. The presentation of the original tomograms often has little diagnostic utility or do not allow certain viewing modes at all. For visualization, it is known to generate presentations referred to as pseudo-3D presentations (three-dimensional visualizations). An example is the method of maximum projection (maximum intensity projection) that is particularly significant for angiography. In this method, the highest intensity value of the gray scale values encountered on a projection beam is selected and imaged into the observer image plane. In the maximum projection, the entire volume is imaged and can be viewed in every direction. Maximum projection is disclosed, for example, by U.S. Pat. No. 5,566,282.
The visualization of the volume data also is possible by means of reconstruction of secondary slices through the data grid. Such a method is the known multi-planar reconstruction. Arbitrarily oriented tomograms are calculated through the data volume. The method is of significance particularly for computed tomography because only tomograms in the transverse direction—or in a direction deviating slightly therefrom given a tiltable gantry—can be generated as original slices, i.e. as primary images. For example, longitudinal sections can be generated from the 3D dataset with multi-planar reconstruction. In multi-planar reconstruction, interpolation is carried out between grid points or voxels for generating a two-dimensional image, for example according to the method of the nearest neighbor interpolation or the method of tri-linear interpolation.
U.S. Pat. Nos. 5,898,793, 4,868,748 and 4,821,213 disclose methods with which boundary surfaces within a 3D dataset can be presented, for example bone and/or tissue boundary surfaces. The boundary surfaces first must be characterized or defined by the operating personnel by means of a limit value or an interval for their density value before a mathematical method then allows a decision to be made as to the affiliation of a particular voxel to a boundary surface, or to a number of boundary surfaces.
The text by Hans-Heino Ehricke, “Medical Imaging: Digitale Bildanalyse und-kommunikation in der Medizin”, Vieweg, 1997, page 87, discloses that planes be defined, and reconstructed from the data volume, and presented as image on the operating console of the image acquisition device or at a separate image workstation for interactive reconstruction of secondary slices. The planes are oriented orthogonally relative to the grid axes, slanting or doubly slanting planes, or completely arbitrarily curved sections. As a result, the structures of interest (for example, optical nerve, spinal column) that are distributed over many images in the original sections can be presented in a single tomogram.
Multi-planar reconstruction is particularly tedious for medical personnel when a secondary section, i.e. an evaluation surface proceeding through the grid, must be defined with high complexity. For example, it is tedious to define or determine a curved evaluation surface in a number of dimensions. Methods that require an input of a parameter (for example, density value) of a specific surface are susceptible to error and/or require experience, or trial and error.